A self-contained breathing closure for flasks and other containers that require gas exchange. An illustrative embodiment of the closure is comprised of a splashguard, an adaptor for attaching the closure to the container, a bellows element and a gas-permeable barrier element. The splashguard is intended to keep liquid contents under vigorous agitation in the container without wetting the gas-permeable barrier. The adaptor couples the closure to the container in a secure fashion. The bellows element allows for repeated changes in the internal volume of the container-closure system. The gas-permeable barrier allows desired gases to enter and leave the container while excluding small particles and/or microorganisms. The technology may be used with existing glass flask technology, or coupled to a plastic flask that may be configured for either single-use, or multiple-use.